Re: proposal: psql: psql variable BACKEND_PID

po 13. 2. 2023 v 18:52 odesílatel Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> napsal:

> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2023-02-13 18:06:23 +0100, Peter Eisentraut wrote:
> >> But what do you need the backend PID for in the first place?
>
> > For me it's using gdb, pidstat, strace, perf, ...
> > But for those %p in the PROMPTs is more useful.
>
> Indeed, because ...
>
> > E.g. I fire of a query, it's slower than I'd like, I want to attach
> perf. Of
> > course I can establish a separate connection, query pg_stat_activity
> there,
> > and then perf. But that requires manually filtering pg_stat_activity to
> find
> > the query.
>
> ... in this case, the problem is that the session is tied up doing the
> slow query. You can't run "select pg_backend_pid()", but you can't
> extract a psql variable value either. If you had the foresight to
> set up a PROMPT, or to collect the PID earlier, you're good. But I'm
> still not seeing where a psql variable makes that easier.
>
> I don't buy Pavel's argument that adding Yet Another built-in variable
> adds ease of use. I think what it mostly adds is clutter. I realize
> that "psql --help=variables | wc" is already 160+ lines, but that
> doesn't mean that making it longer and longer is a net improvement.
>

There are three kinds of variables - there are about 40 psql variables.

I can be mistaken - I thought so somebody if needed filtering in
pg_stat_activity, they can run just "\set"

and he can see

(2023-02-13 19:09:10) postgres=# \set
AUTOCOMMIT = 'on'
BACKEND_PID = 10102
COMP_KEYWORD_CASE = 'preserve-upper'
DBNAME = 'postgres'
ECHO = 'none'
ECHO_HIDDEN = 'off'
ENCODING = 'UTF8'
ERROR = 'false'
FETCH_COUNT = '0'
HIDE_TABLEAM = 'off'
HIDE_TOAST_COMPRESSION = 'off'
HISTCONTROL = 'none'
HISTSIZE = '500'
HOST = '/tmp'
IGNOREEOF = '0'
LAST_ERROR_MESSAGE = ''
...

he don't need to search more

To find and use pg_backend_pid is not rocket science. But use :BACKEND_PID
is simpler.

It is true, so this information is redundant - I see some benefit in the
possibility to see "by using \set" a little bit more complete view on
session, but surely - this is in "nice to have" category (from my
perspective), and if others has different opinion, than we don't need to
spend with this patch more time. This is not an important feature.

Regards

Pavel

> regards, tom lane
>